Beam-expander HUH-moment

Lately i bought a 4x beamexpander. My 60W CO2 lasertube emits a beam of 2.5mm.. At the head, this is still 2.5mm round.
While the hole in the head is 14mm, i thought i could expand the beam to 10mm.
I expected the focuspoint to become 4x smaller ad 16x more density of power.
Well... this point is smaller ( i haven't measured it yet..)
But ... when i engrave, i need much more power than before!
OK, i know i might have some losses in the expander but a max of 15-20% is considered acceptable..
I think the powerdrop is about 90%... :eek:
Is this normal or should i have to reduce my power when everything was normal (like i expected)

Who has experience with beamexpanders??

BTW i focus with a proximityswitch with high accuracy.. my focus is within 0.05mm..

Kees

Are you losing power through scatter on the final lens? remember only 80% of the visible diameter of the final lens is use-able.

best wishes

Dave

You didn't install it backward, did you? :p

@Exsecratio My lens is 20mm so 16mm would be allowed. My beam is ~11mm
@baksiidaa If i had it backwards, my beam should be smaller.. from 2.5 > 10mm would be 2.5mm> 0.8mm right?

Kees

What material are the lenses in the beam expander made from? ZnSe I hope?

Is the beam expander well aligned to the beam?
Is there anything "clipping" the beam so the full beam is not reaching the lens?
Can you remove the lens and see what the shape of the incident beam is? Is it round or a funny shape that would indicate misalignment or clipping?

It would be silly to expect your quoted performance enhancements. Working off theoretical calculations and using low quality optics will not give you agreeable results. You "might" get a little bit smaller focal point and a little more power density but once you are down to a certain level, the quality of your lenses and mirrors really play a huge role.

Good Luck
Matt

Yes, the lenses are ZnSe (yellow)
On the input side i made a bushing that slides over the lasertube's output.
On the other end i made a kind of tripod to align the BeamExpander with the laserbeam.
First i made the alignment without changing the original mirror alignments.
After i was quite close i adjusted the mirrors.

To measure the diameter of the beam i use a 1.5mm thick wooden strip and just burnt through. The outline of the dot is my beamdiameter.. Or is there another way to measure that??
The shapes look quite round to me. (i have seen half-moons during alignment)

Lensquality.. i'm not sure but they look quite clear, compared to medium quality lenses that i received first. they are in the $200 range, chinese..

Kees

The beam expander will also generally correct beam divergence , collimation if you will.
The issue might be that you are introducing another form of convergence / divergence with the expander and you don't see it at the exit , but it will be pronounced over longer paths leading to poor resuts . You need to measure the beam diameter at the focusing optic and at the tube output to see whats happening.

You probably don't have a centered fly path. With a larger beam... It is quite easy to scatter the beam if it is not truly centered.